Pressure dependence of propagation velocity and structure in subcritical 303 GHz argon discharges

Abstract Subcritical millimeter-wave discharges, driven by beams below the classical breakdown threshold, offer promising applications, yet their self-sustained propagation mechanisms remain unclear. We investigate argon discharges driven by a 303 GHz millimeter-wave beam (wavelength λ = 0.99 mm ) over p = 0.1 – 1.8 atm , using high-speed and high-sensitivity imaging. With an incident intensity of 0.17 GW m − 2 (7%–80% of the breakdown threshold), the ionization front propagated at about 200–2000 m s −1 , with the initial velocity scaling as p −1 and the steady velocity as p − 1 / 2 . A Fourier-based image analysis quantified the spatial discharge structure, revealing a persistent sub-wavelength ( ∼ 0.5 λ ) H – k -plane pattern across all pressures, matching 94 GHz atmospheric discharges but differing from the λ -pitch E – k -plane structure in 303 GHz air. These results indicate that the stable propagation mode and filamentary pattern are governed not only by wavelength but also by gas-dependent transport length scales.